The present invention relates generally to flame spray apparatus and to methods of deposition of coatings and bulk materials with thermal spraying technique. More specifically, the present invention relates to high-velocity oxidizer-fuel spraying apparatus and methods. When the oxidizer gas is oxygen, the technique is known as High-Velocity Oxygen-Fuel (HVOF) spraying. Correspondingly, when the oxidizer gas is air or air enriched with oxygen, the technique is known as High-Velocity Air-Fuel (HVAF) spraying.
Thermal spraying is widely used to apply metals and ceramics in a form of coatings or bulk materials on different type of substrates. Majority of thermal spray methods utilizes the energy of hot gaseous jets to heat and accelerate particles of spraying material. When impinging the substrate, the particles form a coating.
Typically, the High-Velocity Oxygen-Fuel (HVOF) apparatus generates a jet of hot gases due to combustion of a fuel and oxygen in internal burner at elevated pressure, usually several bars. The fuel can be gaseous such as propane, methane, propylene, MAPP-gas, hydrogen etc., or liquefied such as kerosene. From the burner the gas expands into an exhaust nozzle or barrel, reaching sonic velocity. Further expansion in atmosphere or into wider section of the nozzle (for instance, Laval's nozzle) results in formation of a supersonic velocity jet. For this reason the technique is named "high-velocity". The first such apparatus was invented by James A. Browning (U.S. Pat. Nos. 4,342,551 and 4,343,605, both issued August, 1982). A number of improvements for the HVOF guns was targeting their better efficiency to heat and accelerate spraying material (U.S. Pat. Nos. 4,370,538; 4,416,421; 4,540,121; 4,568,019; 4,634,611; 4,836,447; 5,019,686; 5,206,059; 5,535,590). In spite of formation of rather dense coatings, the HVOF processes deposit materials with rather high oxide content, as they melt the particles, at least partially, thus making the particle surface very active. As the jets always contain gaseous oxygen due to incomplete combustion and ejection of air from the atmosphere, the particle molten surface oxidizes rapidly. Besides, these processes are rather expansive due to large consumption of compressed oxygen. Finally, clogging of the long nozzle by molten spraying particles creates a lot of problems in operation of the HVOF devices.
The High-Velocity Air-Fuel (HVAF) equipment first was created as a cheaper alternative to the HVOF process (U.S. Pat. No. 5,120,582). The combustion of air and kerosene is the main mode of such equipment operation. The design was further improved in the U.S. Pat. Nos. 5,405,085 and 5,520,334. The main problem of such equipment is unstable combustion at high flow rates of gases as the flame propagation velocity in the fuel-air mixture is two orders of magnitude lower than in the fuel-oxygen mixture. Rather dimensional internal burners are used to stabilize and complete combustion. But this prevents a possibility to introduce spraying powder in the axis of the chamber and nozzle (the chamber is too long for the particles to travel through). The injection of particles into the nozzle side (the alternative to axis injection) creates even bigger problems with the nozzle clogging than in the HVOF equipment utilizing axis injection. The use of hydrogen or methane to ignite the combustion and as a pilot flame in many HVAF guns makes them unpractical and unsafe.
Vladimir Belashchenko and Viatcheslav Baranovski, one of the authors of the present invention did an important improvement in the HVAF design (U.S. Pat. No. 5,932,293). A permeable burner block was introduced into the internal combustion chamber. When heated over the auto-ignition temperature of air-fuel mixture, hot walls and passages of the permeable burner block continuously ignite the mixture, thus stabilizing combustion in rather small-size burner. However, the combustion is stable only in narrow range of air-to-fuel ratio (close to stoichiometrical one) and total gas flow is restricted as it cools the permeable burner block. This significantly restricts variations in technological parameters necessary for spraying of different materials, as well creates technical problems when droplets of liquefied fuel come to the burner (always happens when using propane and heavier fuels). The droplets of fuel evaporate in the burner dramatically decreasing the air-fuel ratio and destabilizing combustion.
Still, the HVAF coatings are rather oxidized, as the combustion temperature (1900 degrees C. for propane-air mixture and higher for heavier fuels) exceeds the melting temperature of majority of spraying materials, thus at least the surface of spraying particles is molten and actively oxidized by oxygen in a jet.
Meanwhile, it becomes clear that the oxidation of materials during spraying can be substantially suppressed when spraying solid, non-fused particles. Anatoley Alkimov and co-authors patented a gas-dynamic spraying method for applying a coating (U.S. Pat. No. 5,302,414). In the method, a supersonic jet is created by expanding of compressed gases having a temperature considerably lower than the fusing temperature of the spraying material. The coating is formed during the impact of solid particles, which are not heated and thus practically not oxidized during deposition. However, the coatings formed of non-heated particles are rather porous compared to the HVOF coatings. The attempts to pre-heat the compressed gas with electrical heaters have little effect on coating quality while making the equipment bulky.
The use of the HVOF and HVAF technique seems to be more promising as the particles can be both accelerated and heated below the melting point of spraying material. James Browning expressed this opinion in U.S. Pat. No. 5,271,965 as well as in Technical Note, published in the Journal of Thermal Spray Technology, 1(4) December 1992, p.289. He assumed that solid particles did undergo fusion during impact with a substrate due to a release of kinetic energy ("impact fusion"). Thus, the particle velocity becomes a key factor of coating formation. The problem is that in known HVOF and HVAF apparatus the reduction of gas temperature in the burner is always accompanied with the decrease of gas velocity. This results in drop of the particle velocity. Rather unusual solutions were found to overcome this problem. For instance, to ensure the particles are in solid state during spraying, J. Browning suggested to cool the jet in the nozzle below the material melting point by injected droplets of water (U.S. Pat. No. 5,330,798). Then, the combustion process itself is not affected. An evident disadvantage of this equipment is a drop of efficiency of the apparatus, as well complication of the process.
Not only the drop of gas velocity with decrease of temperature creates a problem for a good quality coating deposition by spraying of solid particles with the HVOF and HVAF technique. Practically, the necessary drop of gas temperature is impossible to implement in known apparatus without destabilizing the combustion. As known, a stable combustion of oxidizer-fuel mixture at high gas flow is provided in a rather narrow range of oxidizer-to-fuel ratio in the vicinity of stoichiometrical one. The maximal temperature of combustion is reached in this range. For instance, for oxygen-propane mixture the stoichiometrical ratio is about 5:1 by volume and maximal combustion temperature exceeds 2800 degrees C. The excess of oxygen drops the combustion temperature. In case of oxygen-propane, at ratio 2.5:1 the combustion temperature is about 2500 degrees C. Further increase of oxygen content ceases the combustion. In case of air-propane mixture, the stoichiometrical ratio is about 25:1 by volume with maximal combustion temperature about 1900 degrees C. This mixture does not support combustion when the ratio is shifted more than 10% even in presence of "hot" walls of permeable burner block (as in the U.S. Pat. No. 5,932,293). Thus, in the HVAF process, to drop the gas temperature with increase of air or propane content over stoichiometrical is practically impossible. Note that mentioned gas temperatures are well above the melting point of majority of commercial alloys used for thermal spraying. Thus, at least the surface of spraying particles is fused, resulting in rapid oxidation of material during spraying. The use of fine particles, for instance, less than 20 micrometers, aggravates the problem significantly.